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Domestic hot water issue

This discussion was created from comments split from: Bosch Greenstar Combi 151 not able to keep up.


  • Jake6
    Jake6 Member Posts: 2
    I have a Bosch Greenstar FS Boiler Model KWB 42-3. Sometimes the domestic hot water comes on right away then cools down slightly after a couple of minutes then back to hot again after a couple of minutes. Is there an adjustment for this?
  • Burbman
    Burbman Member Posts: 7
    I have the same problem with our Bosch. Sometimes the water will go ice cold for a minute (literally) then come back hot again. I have scoured the web and asked the pros I know and nobody has a definitive answer other than it's just how the boiler is programmed. I am looking at adding an ekectric heater in series after the Bosch to increase capacity and smooth out the temp inconsistencies.
  • neilc
    neilc Member Posts: 1,805
    have these units been serviced? lately?
    burners inspected? and tuned ?

    I had a similar issue a year or 2 after my tankless was installed.
    Either the hot cold hot, or even more fun, hot to cold with lock out.
    I mulch mow my autumn tree leaves back into my lawn, and apparently into the fresh air intake.
    Turned out leaf litter had been drawn into the burner fan, and low fire tubes, and would at times not allow good combustion or primary flame sensor to keep the burner going.
    Once cleaned out, and protective screen added to intake, no more issues.
    something to check on.
    known to beat dead horses
  • neilc
    neilc Member Posts: 1,805
    or it could be water flow,
    or other issues,
    have these units had their annual periodic service ?
    known to beat dead horses
  • Jake6
    Jake6 Member Posts: 2
    Thank you for your comments, I also mulch mow my leaves and have not had my furnace serviced lately.
  • neilc
    neilc Member Posts: 1,805
    another thing,
    mine is oversized, don't ask.
    set it as low a temp as you can.
    Mine is set for 110 or 105, Wife and I like a cooler than hot shower.
    If it's set higher then I mix too much cold in, and fall under my 1/2 gal/min minimum flow rate.
    known to beat dead horses
  • ResqDogz
    ResqDogz Member Posts: 3
    I'm experiencing similar intermittent flow with my Combi 151 domestic hot water side - and suspect a circuit board/relay may be at fault: My pump motor functions fully, but there are inconsistent audible "clicks" followed by the motor stopping... Any suggestions?